The present invention relates in general to a thermal energy storage system, and more particularly, to a refrigerant management control and method for a thermal energy storage system.
In the prior art, certain air conditioning apparatus with thermal energy storage were developed for the purpose of efficiently exploiting the two-tier pricing system utilized by electrical utilities. One exemplary apparatus is disclosed in U.S. Pat. No. 4,735,064 to Fischer.
By way of background, electrical utilities have developed a two-tier pricing structure which is divided into peak hours and off-peak hours. Peak hours occur when electrical demand is maximized, such as those periods of the day corresponding to the average daily highest temperatures, and which generally relate to some extent to those hours surrounding the afternoon-time period. One important reason for the relatively high extent of electrical demand during the period of the day when the temperatures are the greatest (i.e., at the "peak hours") is because of the utilization of air conditioning systems in a large percentage of commercial and residential buildings. The "off peak" hours occur when the outdoor temperatures are cooler and electrical demand is minimized. The "off peak" hours correspond generally to the night time period around and after the midnight hour, when the demand for cooling, is minimized because of the lower outdoor temperature, the relative inactivity of persons, and when the household utilization of electricity for electrical lighting is minimized.
As a result of the greater demand for electricity during the peak hours of the day, the rate prices for electricity during such peak hours are substantially greater than the rate prices for electricity during the off peak hours.
The amount of electricity utilized at business and residential buildings is substantial during peak hours as the condensing unit in the air conditioning apparatus operates to meet the cooling requirements of the building. In view thereof, it has been proposed (such as for example in U.S. Pat. No. 4,735,064 to Fischer and, U.S. Pat. No. 4,637,219 to Grose), that is would be advantageous to store energy during off peak hours and to use such stored energy during peak times to reduce the power consumption of the compressor in the condensing unit.
The prior art structures, as shown for example in the Fischer patent 4,735,064, are directed to apparatus having an insulated storage tank which contains a heat exchanger. The heat exchanger in the storage tank contains a refrigerant. A condensing unit is connected to the heat exchanger for supplying liquid refrigerant to the heat exchanger, which refrigerant upon expansion freezes or solidifies the storage material in the tank during a first time period, which corresponds to the period of off peak electrical demand. The storage medium may be water or a phase change material such as polyethylene glycol. The heat exchanger is also connected to an evaporator which receives cold refrigerant liquid from the heat exchanger in the storage tank during a second time period, which corresponds to the period of peak electrical demand. In addition, the condensing unit is typically connected to the evaporator by means of conduits passing through the storage tank, and thus provides refrigerant to the evaporator during a third time period, when some cooling may be necessary. This third time period occurs during off peak hours. Energy use and operating cost are reduced by operating to provide cooling in this way during off peak hours.
One problem with such prior art structure is that there is melting of some of the ice in the storage tank when the thermal energy storage system is operated during the third time period. Further, operation has proved to be less than optimally efficient due to low evaporating temperature in the direct cooling mode and due to low evaporating temperature operation in the ice making mode to re-make the ice which has been melted during the direct cooling mode operation. Thus, there is an "energy penalty" associated with cooling by the freezing and melting of ice as compared to conventional air conditioning methods involving direct pumping of refrigerant from the condenser of the condensing unit to the evaporator.
An improvement in such prior art is provided by the structures disclosed in application Ser. No. 07,706,057 filed May 28, 1991, and entitled Combined Multi-Modal Air Conditioning Apparatus and Negative Energy Storage System. The said application, which is assigned to the same assignor as the present case, discloses a system which permits optimally efficient operation by means of by-passing of the storage tank by the circulating refrigerant when the apparatus is in the direct-cooling mode, thereby to avoid melting the stored negative heat energy storage medium, usually comprising water which then does not have to be refrozen. The combined multi-modal air conditioning apparatus and negative energy storage system can be operated in a direct cooling mode, an ice making mode and a shift cooling mode to provide improved operating cost efficiency over prior art systems such as that of Fischer 4,735,064. In the direct cooling mode, the ice storage tank is isolated from the refrigeration system. In the ice making mode, the heat exchanger in the storage tank functions as the evaporator in the refrigeration system. Heat is removed from the storage medium. If the storage medium is water, it will solidify and form ice. In the shift cooling mode, the condensing unit is effectively isolated from the storage tank and the evaporator and a liquid pump circulates refrigerant between the heat exchanger in the storage tank and the evaporator.
Neither the known thermal energy storage systems nor the systems disclosed in the copending application Ser. No. 07/706,057 filed May 28, 1991 have not taken into account providing the proper refrigerant change for each mode of operation. The problem was to provide the proper refrigerant charge where it needs to be in each mode of operation and to be able to transport the refrigerant charge to its new location in the system when a switch in mode operation occurs.
An object of the present invention is to provide a thermal energy storage system or TES system with improved refrigerant charge management control and method.
Another object of this invention is to provide a thermal energy storage system operable in direct cooling, ice making and shift cooling steady state modes with two transitory modes that allow for proper control of the refrigerant charge.
Yet another object of this invention is to provide a thermal energy storage system having direct cooling, ice making and shift cooling steady state modes with a hypermigration mode prior to shift to shift cooling and a pump out mode prior to shift to direct cooling in order to properly locate the refrigerant charge in the TES system prior to initiation of each of these steady state modes of operation. Other objects and advantages of the present invention will be made more apparent hereafter.